Nervous System. 549 



When the food reaches the stomach it is in a position requiring the 

 co-operation of the gastric juice which is discharged upon it from the 

 gastric glands of the stomach. These glands are connected with the 

 brain by the pneumogastric nerve, and when this nerve is stimulated 

 there is an increase in the flow of the gastric fluid. Such stimulation 

 takes place whenever food is taken into the stomach, and perhaps through 

 the esophagus on its way down ; but, as stated elsewhere, such stimula- 

 tion may come at second hand, or indirectly, when there is no food to 

 be digested, it being possible that the gastric glands may be stimulated 

 by a " mental state," that is, by the action of associated parts of the 

 brain. Thus, in the case of the hungry dogs, in which the flow of the 

 gastric juice was provoked by a sight of food which they could not get, 

 the stimulation took place in consequence of first a stimulation of the 

 optic lobes of the brain, and next its propagation thence to the pneu- 

 mogastric nerve, and through it to the stomach. The sensation of sight, 

 and the aroused memories of gastric action, constitute the ' * mental 

 state," which forms a part of the chain of stimulation in such case. 



The above is a sample of the hundreds of co-operative reactions which 

 take place in the complicated organic machine, in which the stimulation 

 of every act is a resultant of influences that come from different parts 

 of the body, some near and some remote. The differentiation which 

 finally sdts up one part of the organism to become more sensitive to the 

 stimulation of external energy than the rest, has made some progress in 

 the vegetable kingdom. We had some examples of it in chapter 54. 



The tops of growing plants are sensitive to light, and the tips of the 

 roots are sensitive to pressure and moisture. These two extremities are 

 therefore sense organs, and they have means of nervous communication 

 with other parts, which may be called motor organs. The bending of 

 plants toward the light, which we often observe, is caused by the shorten- 

 ing of the cells on the illuminated side, and if a bending shoot be split 

 down, that side will bend over still more, while the opposite side will 

 tend to fly back. This bending will take place after the plant has its 

 growth, and is not, therefore, in all cases, a growing process. This 

 1 tending is produced by the blue and violet rays. Leaves also twist on 

 their stalks in order to present the green surface to the light. The up- 

 permost part or tip of ( some) seedling plants, is alone sensitive to light, 

 and it transmits an influence to the lower part, causing it to bend. If, 

 therefore, the upper part be wholly protected from light, the lower part 

 may be exposed for hours to it, and yet does not become in the least 

 bent, although this would have occurred quickly if the upper part had 

 been excited by light. As the shortening of the cells, spoken of above, 

 that causes the bending of a stem, takes place at some distance from 

 the point which receives the stimulation of the light, there must be a 



